Many woven and nonwoven webs and fabrics are formed from thermoplastic polymers, such as polypropylene and polyethylene. For instance, spunbond webs, which are used to make diapers, disposable garments, personal care articles, and the like, are made by spinning a polymeric resin into filaments and then thermally bonding the filaments together. More particularly, the polymeric resin is typically first heated to at least its softening temperature and then extruded through a spinnerette to form filaments, which can then be subsequently fed through a fiber draw unit. From the fiber draw unit, the filaments are spread onto a foraminous surface where they are formed into a web of material.
Besides spunbond webs, other fabrics made from polymers include meltblown fabrics. Meltblown fabrics are made by extruding a molten polymeric material through a die to form filaments. As the filaments exit the die, a high pressure fluid, such as heated air or steam, attenuates and breaks the filaments into discontinuous fibers of small diameter. The fibers are randomly deposited onto a foraminous surface to form a web.
During the formation of many polymeric products, such as spunbond webs and meltblown webs, the polymers used to make the products are exposed to various harsh conditions which can adversely affect the properties of the polymers. For instance, during extrusion, a polymer is not only subjected to various external forces, but is also heated to high temperatures. Due to these conditions, the polymers can decrease in strength and elasticity, can become brittle, can yellow or otherwise degrade in color, or can produce an article with a short product life.
Another problem typically encountered when heating and processing polymers as described above is for the polymers to generate smoke. The production of smoke can possibly foul up the equipment, which reduces efficiency and may discolor the polymer.
In the past, various attempts have been made to improve the performance of polymeric resins that are melt processed to form various articles and products. As used herein, melt processing refers to any process, such as a spunbond process or a meltblown process, whereby a polymer is heated and formed into a particular shape. In the past, in order to improve the performance of polymeric resins, additives such as stabilizers have been added to the polymers. For instance, various stabilizers are commercially available that are designed either to prevent degradation of the polymer when exposed to light, to prevent the polymer from discoloring or yellowing, to prevent the polymer from becoming brittle, or for otherwise preserving the properties of the polymer during use.
Unfortunately, typically when a particular stabilizer is added to the polymer to improve a particular property of the polymer, other properties of the polymer may be adversely affected. As an example, phenols are typically added to polymers in order to increase the thermal stability and the process stability of polymers. Phenols, however, can cause the polymers to yellow and have a shorter product life.
Other problems have also been experienced when attempting to combine different stabilizers into one polymer for enhancing the properties of the polymer. In particular, many stabilizer products are incompatible. For instance, when combined together, stabilizers can render each other ineffective or, instead of improving the performance characteristics of the polymer, can adversely affect the polymer. In fact, since many stabilizers are relatively complex chemical compounds, the results and affects that will occur when different stabilizers are combined is very unpredictable.
One particular problem experienced in the past has been the ability to add to a polymer a stabilizer that will increase the thermal aging stability of the polymer without compromising other properties of the polymer. The thermal aging stability of a polymer refers to the ability of the polymer to resist degradation when exposed to high temperatures for an extended period of time. When exposed to high temperatures, most polymers become brittle and will ultimately lose most of their strength and elasticity.
For polymeric products that are exposed to high temperatures during use, it is important for the polymers to have a substantial amount of thermal aging stability. For instance, woven and nonwoven polymeric fabrics used in the health care industry are typically sterilized prior to use by being placed in a steam oven for a set period of time. For these products, it is important that the polymers do not substantially degrade during the sterilization process.
As stated above, however, in the past, it has been found to be very difficult to increase the thermal aging stability of a polymer without adversely affecting other properties of the polymer. For instance, when stabilizers have been added to polymers for increasing the thermal aging stability, problems have been experienced with yellowing, with the polymer producing smoke during melt processing, and with the ability to maintain the meltflow rating of the polymer within preset limits. Significant problems have also been experienced in attempting to combine a stabilizer that improves the thermal aging stability of the polymer with other stabilizers. Stabilizers used in the past for increasing the thermal aging stability of a polymer have been found to be incompatible with many other additives and stabilizers.
In view of the above deficiencies of the prior art, a need currently exists for a stabilizer or a combination of stabilizers that will significantly improve more than one property of a polymer without any adverse side effects. In particular, it would be very desirable if a stabilizer formulation could be created that provides stability to the polymer during melt processing, that prevents smoke formation during melt processing, and that makes the polymer more thermally stable when exposed to high temperatures for an extended period of time.